The present invention relates to a frequency comparing circuit which compares two frequency signals.
Recently, devices which perform speech synthesis using digital technology have been developed and used. In such a device, an impulse and a white noise are used as a sound source. The signal from the sound source is passed through several digital filters to obtain an audio signal. The conditions for the digital filters depend on the audio signal to be achieved. Furthermore, the conditions for the digital filters in the digital speech synthesizer are set by analyzing and recognizing an actual voice.
FIG. 1 is a conventional block diagram of a speech recognition circuit. In the figure, a mike amplifier 1 amplifies an analog signal from a microphone (not shown). The output from the mike amplifier 1 is supplied in parallel to, for example, four band-pass filters (BPF) 2A, 2B, 2C and 2D. The signals which pass through the four band-pass filters 2A, 2B, 2C and 2D are detected by four detectors, respectively. The detected four signals are supplied to four low-pass filter circuits 4A, 4B, 4C and 4D, respectively. The signals which pass through the low pass filter circuits 4A, 4B, 4C and 4D are selectively supplied to an analog/digital converter (A/D) 6 through a multiplexer 5. The digital output from the analog/digital converter 6 results in a recognition result for the voice input through the microphone.
The employment of switched-capacitor filter technology introduces higher integration and higher accuracy to the recent speech recognition circuit. In other words, the mike amplifier 1, band-pass filter circuits 2A through 2D and low-pass filter circuits 4A through 4D all adopt the switched capacitor circuit. For the control of the switched capacitor circuit, any circuit which uses such a switched capacitor circuit is required to include an oscillator and a clock generator which produces various clock pulses from the oscillator output.
When an accurate oscillating frequency is required, the actual oscillating frequency can be matched to the accurate oscillating frequency by knowing whether the actual oscillating frequency is greater or smaller than the accurate oscillating frequency, or the fluctuation or ratio between the actual oscillating frequency and the accurate oscillating frequency.
A PLL (phase locked loop) circuit has been known as a means for accurately detecting which frequency is greater or smaller, or the fluctuation or ratio between the actual and accurate frequencies.
However, the PLL circuit is not adapted to integration due to its complicated construction. Therefore, with prior art circuits with simplified constructions, an accurate oscillating frequency can not be obtained, thereby degrading the switched capacitor circuit which leads to a speech recognition circuit with poor accuracy.